In fixed bearing tri-component implants, such as knee or ankle implants, the locking interface between the polyethylene bearing construct and the metal base construct can be a significant source of wear debris. More particularly, the sliding motions (or micromotion) in the junction between the polyethylene bearing construct and the metal base construct produce polyethylene particles that can migrate into the body. Small abrasive particles can also migrate into the interface between the polyethylene bearing construct and the metal base construct and scratch the metal base construct, particularly when the metal base construct is formed out of titanium. This issue of “backside wear” has generally been a long-term problem in tri-component implant devices such as total knee arthroplasty (TKA), total ankle arthroplasty (TAA), spinal implants, and bi-polar hip implants. For instance, the backside wear in TKA occurs at the interface between a surface of a tibial insert and a surface of a tibial tray and at the interface between a surface of an intermediate bearing component and a surface of a tibial component in TAA.
Particles caused by backside wear may cause osteolysis and other degenerative conditions. These particles may further act as an abrasive and accelerate wear over time. Efforts have been made to reduce backside wear effects by using more wear-resistant cross-linked polyethylene (XLPE) inserts, which are generally more brittle and stiffer than conventional polyethylene inserts. As such, XLPE inserts may be disadvantageous in high impact applications, such as knee and ankle replacement applications. Other attempts to address the backside wear issue also include polishing techniques such as polishing the titanium mating surface of the metal base construct, e.g., tibial trays or tibial components. While polishing techniques may reduce backside wear, these techniques may not significantly reduce the number of wear particles created. Other attempts to reduce backside wear have focused on refining and improving locking features (e.g., dovetail grooves) to better secure and restrain the tibial insert from moving relative to other fixed components, e.g., the tibial tray. The basis for those attempts included the theory that by preventing micromotion, backside wear could be minimized and wear particles would not occur. However, such efforts have made connecting the insert to the tray in a surgical procedure more difficult, because such locking features generally operate under very tight tolerances. Soft tissue, blood, and bone chips may interfere with the tight tolerances of such locking features and may make assembly very difficult and time consuming. If any wear particles or hard biological matter (e.g., bone chips) do end up between the tight fitting locking mechanism components, wear rate may be further accelerated. In short, backside wear remains a problem as the prior art has failed to completely eliminate micromotion. These attempts have left surgeons frustrated with tight-fitting inserts.
In mobile bearing implants, there are also other problems caused by the generally free movement of the inserts as compared to the fixed components in fixed bearing implants. Mobile bearing components may be used with uni-compartmental, bi-compartmental, or tri-compartmental prosthetic devices and are thought by some to provide a “natural” movement, “natural” feeling, and/or also serve as “self-adjusting” means to compensate for slight misalignment. However, in most cases, mobile bearing components comprise less than ideal wear couples. For example, a typical mobile bearing of the prior art may comprise a tibial tray made from polished titanium or stainless steel, and a tibial insert made of conventional or cross-linked polyethylene. When the tibial insert articulates against the tray, a wear couple is formed. Such mobile-bearing wear couples of the prior art are not ideal because titanium and/or stainless steel do not have superior bearing properties when used with polyethylene or articular cartilage. Similarly, the prior art mobile bearing ankle prostheses suffer from such non-ideal coupling of the components.
There are also disadvantages with McKeever and McIntosh-style tibial hemi-arthroplasty devices that have been provided in the prior art to address problems involving unicondylar knee replacements performed in patients having a femur which may not need replacing. In particular, these devices are made from conventional materials (e.g., titanium, stainless steel, Vitallium metal) which are less than ideal for articulation with both polyethylene and natural articular cartilage. See Springer et al., “McKeever Hemiarthroplasty of the Knee in Patients Less Than Sixty Years Old,” J Bone Joint Surg Am. 2006, 88:366-371.
In view of the above, there exists a need for tri-component implants, whether fixed bearing or mobile bearing, with strengthened, low friction, highly wear resistant surfaces that significantly reduce backside wear, and provide more ideal coupling between the components and with improved bearing surface properties with natural articulating cartilage.